Field of the Invention
The invention concerns a method for magnetic resonance imaging by operation of a magnetic resonance apparatus, as well a magnetic resonance apparatus and a non-transitory, computer-readable storage medium encoded with programming instructions to implement such a method.
Description of the Prior Art
In a magnetic resonance apparatus, also called a magnetic resonance tomography system, the body of a person to be examined, such as a patient, is normally exposed, with a basic field magnet, to a high main magnetic field, for example of 1.5 or 3 or 7 Tesla. In addition, gradient fields are activated by a gradient coil unit. Radio-frequency pulses, in particular excitation pulses, are then transmitted via a radio-frequency antenna unit using suitable antenna devices, that cause nuclear spins of particular atoms to be resonantly excited by these radio-frequency pulses are deflected by a defined flip angle relative to the magnetic field lines of the basic magnetic field. As the nuclear spins relax, radio-frequency signals, known as magnetic resonance signals, are emitted, and are received using suitable radio-frequency antennas and then further processed. Finally the desired image data can be reconstructed from the raw data acquired in this way.
For a particular measurement a specific magnetic resonance sequence, also called a pulse sequence, must be transmitted, which is composed of a sequence of radio-frequency pulses, in particular excitation pulses and refocusing pulses, as well as gradient activations that are appropriately coordinated therewith, and proceed in gradient axes along respective spatial directions. Timed to coincide with this, readout windows are set that define the time periods in which the induced magnetic resonance signals are detected.
The gradient activations of a magnetic resonance sequence are typically defined with respect to gradient amplitude, gradient pulse duration and edge steepness, in other words the first derivative with respect to time dG/dt of the gradient amplitude, normally called the slew rate. In most magnetic resonance sequences, the polarity and strength of the gradients have to be changed at extremely short time intervals. During the switch-over process there is certain distortion and/or expansion of the magnetic coil, so that loud noises arise and patients need noise protection.
In particular the gradient switchings of an echo-planar imaging magnetic resonance sequence, which is typically used in examinations of the head, but also in the abdominal region of the body, have a very high slew rate, so that the execution of the magnetic resonance sequence is typically very loud. Furthermore, strong vibrations of the magnetic resonance apparatus can occur during the execution of a magnetic resonance sequence using echo-planar imaging. The gradient switchings with high slew rates are in this case used in particular during the readout windows of the magnetic resonance sequence. Patients with claustrophobia or who are fundamentally uncomfortable with the magnetic resonance apparatus may in addition be alarmed by loud examinations and/or unknown noises, so that an examination can only be carried out by administering tranquilizers, or is refused outright.